Some mobile devices have features for determining a geographic location. For example, a mobile device can include a receiver for receiving signals from a global positioning satellite system (e.g., global positioning system or GPS). The mobile device can determine a geographic location, including latitude and longitude coordinates of the device, using the received GPS signals. In many places, GPS signals can be non-existent, weak, or subject to interference, such that it is not possible to determine a location accurately using the GPS functions of the mobile device alone. For example, a conventional mobile device often fails to determine a location based on GPS signals when the device is inside a train traveling underground in a subway system.